1. Field of the Invention
The present invention relates to composite lens assemblies in general, and in particular, to such assemblies and methods for assembling in which plastic lens elements are adhesively bonded to other plastic lens elements or to glass elements.
2. Description Relative to Prior Art
Image forming plastic lens elements have, of course, been used for many years. Sometimes a single plastic lens element is all that's required. Other requirements dictate that a combination of two or more plastic elements must be used in a lens design to correct for Seidel and chromatic aberrations, and to meet other lens requirements. In addition to this, in multi-elements lens systems, it is often desired to cement two elements together and form what is commonly called a cemented doublet. Cemented doublets are desirable when certain lens designs require that two lens elements be placed in intimate contact or almost intimate contact. Cementing the two elements maintains alignment and more importantly reduces the reflections at the two surfaces to be cemented. The result is an increase in transmission and subsequent improvement in image contrast.
The design of plastic lens systems has been limited, in part, by the inability to cement plastic doublets in production. Where such assemblies require bonding of the plastic elements or a plastic element to a glass element by an optical cement, difficulties sometimes arise in establishing or maintaining a physically firm and stable bond between the plastic element and the glass element or between the plastic elements. That is to say, the plastic and the cement are incompatible in the sense that sooner or later any bond initially established between them may be adversely affected in a manner impairing its optical neutrality. For instance, while as a rule stable bonds may be established between glass elements and optical cements, it has been found that the bonding of plastic elements by optical cements to glass elements or other plastic elements is often impaired by partial or total separation, crazes, fissures or cracks in the cement, etc., for reasons sometimes rooted in their different coefficients of expansion under conditions of changing temperatures or, generally, because of their lack of compatibility. Much recent work had been devoted to special cements that are suitable for cementing plastic surfaces to each other. U.S. Pat. No. 4,690,512 represents this trend or approach. The invention establishes a different approach to the solution of the above-mentioned problems.